Electric amplifier



G. L BEERS Feb..f 28, 1939,

ELECTRIC AMPLTFTER Original Filed Aug. 13, I5 Sheets-Sheet l w r n.

ATTORNEY 3 Sheets-Sheet EQ E EY E R ATTORNEY G. I. BEERS ELECTRIC AMPLIFIER Feb. 28, 1939.

Original Filed. Aug. 15,

' Feb. 28, 1939. G, L, BEERS' Re. 21,014

ELECTRIC AMPLIFIER Original Filed Aug. 13, 1927 3 Sheets-Sheet I5 Fig.5.

n4 j l I'l Z INVENTOR ITNIQESSES: I W George LBeers.

' v ATTORN Y Reissued Feb. 28, 1939 a Re. 21,014

Emo'rmc AMPLIFIER George L. Beers, Collingswood, N. J., assi gnor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania Original No. 1,960,723, dated May 29, 1934, Serial No. 212,791, August 13, 1927.

Application for reissue May 28, 1936, Serial No. 82,383

23 Claims.

My invention relates to electric amplifiers, and it has particular relation to such amplifiers utilized in connection with the reception of radio signals and signals transmitted over wire lines by means of carrier currents.

The reception of radio signals, and of signals transmitted by carrier currents over line wires, popularly known as wired-wireless, is often affected by the phenomenon known as fading.

m The fading may be caused by fluctuations in the attenuation of the signal in the transmitting medium, or, as it is sometimes stated, it may be ,caused by variations in the heightof the so-called Heaviside layer. Irrespective of the actual cause of fading, however, it results in a very annoying variation in the amplitude of the ultimate sound output of a receiving set, at times causing the signal to decrease almost beiowaudition, and at other times causing the loud speaker to be overloaded.

It is accordingly one object of my invention to provide, in an amplifying system, instrumentalities and circuit connections therefor, whereby fluctuations in the amplitude of an incoming signal shall be compensated.

Another object of my invention is to provide an automatic volume control for an amplifier that shall compensate variations in the input-voltage to any stage thereof.

Another object of my invention is to provide, in radio receiving apparatus, automatic means for limiting the effect of a powerful local station on the loud speaker, or other sound-producing instrumentality, if the apparatus is inadvertently tuned to the carrier frequency of said local station during the operation of tuning-to receive distant stations.

Another, and more specific, object of my invention is to provide, in anamplifier system, means for changing the volume control thereof from automatic actuation to manual actuation.

To attain the aforementioned objects, I provide in one embodiment of my invention, an amplifier with an auxiliary volume-control device in the nature of a thermionic tube having its grid so negatively biased that itfunctions as a detector. The plate, or output, circuit of the control device comprises a resistor having a value in the neighborhood of .5 megohm shunted by a condenser of about 4 microfarads capacity. and

the said resistor is included inthe input, or grid circuit of the tubes that are to;be controlled.

The resistor and the condenser form a network having such a time constant that the audio-frer.5 quency component of the detector tube ouput develops no voltage across it, such voltage being developed only by components below audibility, or, in other words, components corresponding to the usual rate of fading. The fading rate, in general, varies from one cycle in five minutes to 5- approximately five cycles per minute. A connection is provided between an appropriate point in the amplifier circuits and the grid of the control tube, wherebya portion of the potential of an incoming signal may be impressed thereon. The 10 changes in the resistance, or IR drop in the output resistor, caused by changes in the average amplitude of the signal, are, by suitable connections, applied to the grid of the amplifier tube controlled thereby, or to the grids of a plurality 16' of amplifier tubes, in orderv that a signal which is increasing in strength will meet an amplifier having less effective amplification than normal, and a signal which is decreasing in strength will meet an amplifier having more eflective amplifi- 20 cation than normal.

In order that the output of the amplifier may be iau'ijusted to a predetermined level,. and may be maintained fixed at such level, the grid of the control-device may be provided with appro- 25 priate biasing means to fix its normal potential with reference to the potential applied thereto from an incoming signal. If, for example, the voltage for the actuation of. the control-device,

is derived from the input circuit of a power-tube 3 used as a second detector, in which the voltage for a desiredout'put level should not be greater than approximately thirty volts, I have found that good results are obtained by biasing the control-device grid to a potential of approximately 35 thirty volts. Specifically, in the case just referred to, the bias applied to the-control-device grid would be thirty volts more negative than the cutofi potential, thus preventing the said controldevice from functioning until the voltage on the 40 second detector exceeds thirty volts, and preventing it from drawing grid current until the said voltage exceeds thirty by an amount substantially equal to the cutoif potential.

For higher output levels the voltage on the 45 control device gridis made more negative than in the example just given, and for lower output levels it is made less negative. In any event, the grid is given a potential that is more negative than the cutoff potential, the increase in the negative 5 direction being approximately equal to the voltage on the second detector grid which gives rise to the output level desired. Such an arrangement permits the use of a rel.- atively low potential of approximately ninety analogous to the well known Radiotron 201-A may accordingly be utilized therefor.

Among the features that I consider characteristic of my invention are those set forth with particularity in the appended claims. The inven-.

A invention that may be utilized under certain conditions, as, for example, when it is desired to control a thermionic tube handling a considerable amount of power, or where'the voltage available for control purposes is insufficient.

Fig.' 3 is a diagrammatic view of a complete receiving system of the superheterodyne type, illustrating the application thereto of a preferred form of my invention, and

Fig. 4 is a diagrammatic view of a modified form of my invention, wherein the control-voltage is obtained directly from the plate circuit of the second detector-tube in a superheterodyne receiving system.

Figure 5 is a diagrammatic view of my invention showing the automatic volume control as applied to amplifiers preceding and following the tube from which the controlling potentials are derived.

My invention is applicable to substantially any stage of a radio frequency amplifier, or substantially any stage of an intermediate frequency amplifier, such as is found in receiving sets of the superheterodyne type. It is also feasible to derive the control potentials for my device from any one of the above-referred to stages. For this reason, in order to simplify the explanation of my invention, I have shown my invention in Fig. 1 associated with appropriate circuit connections for applying it to any desired amplifier stage. In the actual practice of my invention, the majority of the connections shown in Fig. l are eliminated, only those being retained which serve to control the appropriate stage, or stages.

Referring specifically to Fig. 1, a thermionic device I having a filament 2, a grid 3 and a plate 4 is comprised in an amplifying stage. The input circuit for the thermionic device comprises an inductor 5, shunted by a tuning condenser 6. The filament 2 is energized from a battery 'I and plate potential is supplied from battery 8, connected in series with an inductor I shunted by a tuning condenser I I, a common connection of the inductor and condenser being connected to the plate 4 of the thermionic device.

An inductor I2 having a condenser I3 connected in shunt thereto is coupled to the output inductor ID and is comprised in the input circuit of a suc ceeding amplifier stage.

An inductor l4 having a tuning condenser I5 connected in shunt thereto is coupled to the input inductor 5, the inductor I4 being comprised in the output circuit of an antenna ora preceding amplifier stage. A C-battery I6 for appropriately biasing the grid has its positive pole connected to the filament 2 and functions in a manner which will later be described in detail.

A switch arm 2|] cooperating with a plurality of fixed contacts 2|, 22, 23, 24, .25, 2.6, 21, 28, and 30 21,014 volts on the plate of the control-device, and a tube is provided. The contact 2| may be connected to any appropriate point in the stages of the amplifier preceding the thermionic device I by means of a conductor 3|. The fixed contact 22 is connected by a conductor 32 to the output circuit which comprises the inductor I4 and the tuning condenser IS. The fixed contacts 23, 24 and 25 are connected to separated points on the input inductor 5 by means of conductors 33, 34 and 35 respectively. The fixed contact 26 is connected by means of a conductor 36 to a movable arm 31, which makes contact with a resistor 38 connected in shunt to the input inductor 5. The fixed contact 21' 2: connected to the plate of the thermionic tube by means of a conductor 40 or may be connected to an intermediate point on the inductor Ill. The fixed contact 28 is connected in a similar manner to the input inductor l2 by a conductor 4|, 'or it may be connected to an intermediate point thereon. A conductor 42 is connected to the fixed contact 3|) and extends to an appropriate point in a stage of the amplifier succeeding the thermionic device I.

One end of the input inductor 5 is connected througha bypass condenser 43 and a movable contact 44 to any desired point on the C-battery I6, whereby the initial sensitivity of the receiver may be controlled,

A control device 45, preferably'a thermionic tube having afilament 46, a grid 47, and a plate 48, is provided with an output circuit comprising a B-battery 56, and a resistor 5|, the B-battery being adjustably connected to the plate by means of a movable contact device 52. The filament is energized from a separate potential source 53.

The positive pole of a C-battery 54, is connected to the filament and is adjustably connected to the switch arm 20, through a movable contact device 55, a variable resistor 56, a condenser 5'! and a conductor 56.

The grid 41 is adjustably connected to the resistor 56 by means of a contact device 60.

The end of the resistor 5| which is connected to the filament 46 of the control tube, is also connected by means of a conductor 6| to a switch arm 62 with which are associated a plurality of fixed contacts 63, 64 and 65. The fixed contact 64 is connected by means of a conductor 66 to the contact device 44 associated with the C-battery IS. The fixed contact 63 is connected by a conductor 61 to an appropriate point in the grid or input circuit of'a preceding amplifying stage. The fixed contact 65 is connected by a conductor 68 to an appropriate point in the input or grid circuit of a succeeding amplifier stage;

A movable device ID is associated with the resistor 5| and is connected by means of a conductor H to a movable switch arm I2 with which is associated a plurality of fixed contacts 13, I4 and 75. The fixed contact 14 is connected by a conductor 76 to the lower end of the input inductor 5. The fixed contact 13 is connected by a conductor TI to an appropriate point in the input circuit to a preceding amplifier stage, while the fixed contact I5 is connected by a conductor 18 to an appropriate point in a succeeding amplifier stage. The switch arms 62 and I2 may be linked together for simultaneous actuation as shown by a dotted line I8 and a switch 80 may be provided to short-circuit the condenser 43, when the control is applied to diiferent amplifier stage.

In order that the operation of the sample form of my invention illustrated in Fig. 1 may be made the switch arm I! is associated with the fixed contact 14. When in this position, a. variable portion of the resistor 5| is connected in shunt to the bypass condenser 43 and is, accordingly, included in the input circuit of the thermionic device I, It is, of course, obvious that by proper manipulation of the linked switch arms, the said resistor may be associated with the input circuit of either a preceding or a succeeding amplifier stage, or by a slight modification with both preceding and succeeding stages in which event the condensers 8| and 82 are analogous to the bypasscondenser 43, which may be short-circuited as described previously.

Figure 5 illustrates the modification discussed in the preceding paragraph wherein the volume control efi'ect is applied to preceding and succeeding stages with respect to the tube from which the control potentials are obtained. The circuit illustrated has been based on the circuit of Fig; 1, and similar reference numerals apply to corresponding and equivalent elements in both circuits. It.is understood, of course, that the same modification may be embodied in any of the other circuits illustrated in the remaining figures of my drawings. It will be apparent from an inspection of the circuit of Fig. 5 that the volume control biasing potential derived from the resistor 5| in the plate circuit of the volume control tube is applied simultaneously to an amplifier I14 preceding the tube from which the control potentials have been derived and to an amplifier I15 following the tube from which the control potentials have been derived.

Although the basic principles underlying the action of the control-tube remain the same irrespective to which portion of the receiving system the control-voltage is applied, it is interesting to note at this point that the results obtained by applying the voltage to a tube ahead of the tube from which the voltage for the control-tube is derived differ from the results obtained when the control is applied to a tube later in the series. In the first case, the ultimate audio-output of the system will not be reduced below a certain predetermined level, no matter how strong the incoming signal may be, while in the second case the various potentials may be so adjusted that a strong signal from a local station will not actuate the loud speaker if the circuits of the syst m are tuned exactly to the carrier-wave of such station, while weaker signals from distant stations may be received at normal volume.

It is also assumed for purposes of explanation,-

that the switch arm 2|) is resting upon the fixed contact 23 as indicated by the dotted lines. When in this position, the resistor 56 associated with the control tube 45 is connected through the condenser 51 to a point on the amplifier stage including the thermionic device I which assumes the highest radio frequency potentials.

the voltage of the C-battery l6 adjusted in order that the grid 3 of the amplifying tube I may be maintained at the best D. C. potential for eflicient radio frequency amplification. The bias on the grid of the control tube 45 is adjusted to substantially the cutofi point, i. e., to the poplitude.

tential at which the tube is practically non-conductive. This point is preferably chosen at the lower or negative end of the characteristic curve of the tube. when so biased, the control tube functions as a detector, an incoming signal causing an increased flow of current in the output circuit thereof, which includes the resistor 5|.

If desired, the C-battery associated with the amplifier tube may be eliminated,-in which case the tube is operated at what is known as zero grid-potential.

By suitably adjusting the normal grid bias on the amplifier tube I, and by properly varying the amount of the resistor 5| connected in shunt to the condenser 43, the amplifying stage may be applied to the grid of the control tube tends to cause an increase. in the average current flowing through the resistor 5|. This increase inthe average current causes a greater IR, or resistance, drop in the resistor 5| and the point to which the switch arm 12 is connected consequently assumes a more negative potential with respect to the point connected to the switch arm 62 than it had previous to the increase in signal am- The grid 3 of the amplifier tube I being connectd to the said point on the resistor is therefore caused to assume a more negative average potential'than beforeand the effective amplification of the tube consequently decreases. It is thus apparent that when an incoming signal tends to overload the amplifier stage such tendency is immediately compensated by the applicationof a more negative potential to the grid of placed in condition to efiiciently amplify incomthe thermionic device with reference to the filament thereof.

It has also been found feasible to so arrange the device that a signal of an amplitude which ,causes the desired sound output from the ultimate stages of the amplifier is just sufficient in amplitude to cause enough drop through the resistor to keep thethermionic amplifier tube biased to an intermediate point on its effective amplification curve. A decrease, therefore, in signal amplitude will result in a more positive bias being applied to the amplifier stage and consequently an increase in the effective amplification thereof,while an increase in the amplitude of an incoming signal will so bias the said stage, as was previously explained, as to decrease its effective amplification.

It is also possible to reverse the connections to the resistor 5| shown in Fig. 1, soarranging the system that anincrease in amplitude of the incoming signal causes the grid of the amplifying tube to swing sufiiciently positive with respect to the filament thereof toobtain practically the same results as if the grid were caused to swing more negative.

In certain receiving systems, the strength of signal derived from the amplifier is insufficient to properly actuate the 'control tube. In such case, a high frequency amplifying stage may be interposed between the amplifier proper and the control tube or the system may be arranged in a manner analogous to that illustrated in Fig. 2.

In the latter figure, the control tube with its associated batteries, circuit connections, etc., is shown as being replaced by a detector-amplifier system. The elements of Fig. 2 which are analogous to those of Fig. 1 are similarly numbered.

The output resistance 5| instead of being directly connected to the linked switch arms 62 and I2 is connected between the grid 90 and the filament 9| of a second thermionic device 92, which is supplied with filament power from a separate source 93. The output circuit of the thermionic device 92 comprises a source of potential 94 and a resistor 95 analogous to the resistor 5i shown in Fig. 1. With this resistor are associated the switch arms 62 and 12 together with the cooperating fixed cont acts.

The operation of the modification shown in Fig. 2 may be explained by assuming that the incoming signal which is tending to increase, or is-increasing, in amplitude, is applied to the .grid 41 of the control tube through the conductor 58 and the resistor 56. The control tube, being biased to substantially the cut-off point, functions as a detector, in a manner well-known to those skilled in the art. The incoming signal, therefore, causes an increase in the current flowing in the output circuit thereof including the resistor 5i, causing the end of the resistor 5i associated with the C-battery 95 to become more negative with respect to the point on the resistor connected to the grid 90 than it was previously to the signal increase. The current in the resistor 95 included in the output circuit of the tube 92, consequently increases, and the switch arm 12, which is connected tothe grid of the appropriate amplifier stage, becomes more negative with respect to the switch arm 52, connected to the filament of the same stage, than it was previous to the signal amplitude increase. The tube 92 may be of considerably higher power than the control tube 45, the IR or resistance, drop in the resistor 95 may thus be made considerably greater than the dropin the analogous resistor 5| shown in Fig. 1.

The system illustrated in Fig. 2 has proved to be of considerable advantage in a receiving set of the type in which the high-frequency voltage developed is insufficient to properly actuate the system shown in Fig. 1.

My invention is capable of being applied -to substantially any radio-receiving system known to the art at present, including systems in which equi-potential cathode tubes are utilized. To illustrate however, the numerous variations in form which my invention may'take when applied to a variety of different radio receiving systems would necessitate a. large number of drawings and would require a great deal of repetitious explanation. I have; therefore, chosen for purposes of further description, the application of a preferred form of my invention to a receiving systemof the superheterodynetype, in which a common voltage source is utilized to supply plate potential and grid bias to both the amplifier tubes and the control tube.

Referring specifically to Fig. 3, a radio receiving set of the superheterodyne type comprising a radio-frequency amplifying stage I00, a first detector IOI, an intermediate, or beat, frequency amplifying stage I02, a second detector I03, an audio frequency amplifying stage I04 and an oscillation generator I05, is illustrated. Each of the stages shown, with the exception of the first and second detector could, in practice, comprise a plurality of tubes, but in order to simplify the diagram the amplifying stages are illustrated as comprising only a single tube each.

Inasmuch as the operation of the control tube is greatly facilitated by the utilization of relatively high input-voltages, it is desirableto provide a source of such voltages in the receiving system. Perhaps the most convenient method is to employ as a second detector a tube of the power-type, capable of handling an input voltage swing of as much as thirty volts, and deliving sufiicient power to enable the discarding of one of the two audio-frequency amplification stages employed previous to my invention. In addition to permitting the building up of voltages sufiiciently high to satisfactorily operate the control-device, the power-detector gives an output of suflicient amplitude to enable a single stage of power audio-frequency amplification to actuate, at maximum efficiency, all sound reproducing devices that previously required the use of several stages of audio-frequency amplification.

The input circuit of the first radio-frequency amplifying device may comprise a loop I06 tuned by a condenser I 01. The stage may be [neutralized, if desired, according to the teachings of the Rice Patent No. 1,334,118, by means of a condenser I08 connected between the output circuit therof and one end of the loop.

Plate potential for the radio frequency amplifier I00, the first detector IM and the intermediate frequency amplifier I02 is supplied through a conductor III] which is connected to an appropriate point on a voltage source III. Plate potential for the second detector I03 is supplied through a conductor II2 from a different point on the voltage source, and plate potential for the audio amplifier is supplied from a still diilerent point on the same source, a loud speaker II3 being interposed in the last named connection.

The filaments of all of the tubes included in the amplifier proper are supplied from the same source (not shown) one terminal of which is connected to a common grounded conductor Ill. The grounded conductor I is connected to an intermediate point on the voltage source I I I, the point of connection being assumed to be at zero potential. r

The grids of the thermionic devices comprised in the radio frequency amplifying stages and the intermediate frequency amplifying stages are connected by means of conductors H5 and H6 respectively to a second conductor I I! which terminates in a movable contact device I I8 associated with a resistor I21, analogous in some respects to the resistor 5| shown in Fig. 1.

Grid biasing potential for the first detector tube is supplied from a point on the voltage source III which is approximately 9 volts negative with respect to the filament of the tube. Grid biasing-potential for the second detector tube, which is of the power type, is supplied through a conductor I20 from a point on the voltage source III still more negative with respect to the filaments, and the audio frequency power-amplifier stage is supplied with grid biasing potential through a conductor I2I from a point? more negative than the point which supplies the second detector grid bias.

The output circuit of the control tube I22 comprises resistor I21, one end of which is connected to the plate I25 and the other end of which is connected to the zero, or grounded, point on the voltage source III by means of a conductor I28. The filament of-the control .tube is connected by a conductor I30 to a point on the voltage source which is 90 volts negative with respect to the zero point. This method of connection serves to impress a positive potential of 90 volts upon the plate of the control tube with respect to the filamerit thereof through the resistor I21.

The grid I24 of the control tube is connected to one end of the resistor I26, and the opposite end of the said resistor is connected to the filament I23 through a by-pass condenser I3I, and is also connected by a conductor I32 to a point on the common voltage source which is 103 volts negative with respect to the zero, or ground point, thereon,

The grid of the control tube, accordingly, is normally biased to a potential approximately thirteen volts negative with respect to the filament thereof.

A conductor I29 is connected between the plate I25 and a point on the voltage source III approximately 20 volts negative with respect to the zero point, and interposed in this conductor is a. switch device I33. 'A connection I34 extends between the input resistor I26 and the grid of the second detector tube, a coupling condenser I35 being interposed therein to prevent shortcircuiting a portion of the voltage source I I I.

The normal operation of this modification is substantially the same as the operation of that modification of my invention shown in Fig. 1, if the switch I33 is open. The grids of the thermionic devices in the radio frequency amplifier stage and the grids of the thermionic devices in the intermediate frequency amplifier stage are connected to a point on the output resistor I21 by means of the contact device H8. The lower end of the output resistor is connected by the con-.

ductors I28 to the filaments of these tubes. Consequently, a variable portion of the resistor I2l' may be included between the grids and the filaments of the radio frequency and intermediate frequency amplifier tubes and the potential of the said grids with respect to the said filaments is thus determined by the IR or resistance drop in the said resistor.

By reason of the fact that the grid of the control tube is normally given a negative bias of approximately 13 volts with respect to the filament thereof, the tube conducts practically no current in the absence of an incoming signal. The amount of .this current can, of course, be regulated by regulating the plate potential as well as the grid biasing potential.

When an incoming signal is impressed on the second detector tube, it is also impressed on the grid circuit ofthe control tube. An increase in the average amplitude of such signal causes an increase in the output current of the control tube which flows in the output resistor thus serving to give to the movable arm II8 a more negative potential with respect to the conductors I28 and I I4 than it had previous to such amplitude-increase. Inasmuch as this movable arm is connected to the grids of the radio frequency and intermediate frequency amplifier tubes and the ,conductor I I4 is connected to the filaments thereof, the said grids will accordingly acquire a more negative bias with respect to the filaments when the signal amplitude increases.

The system illustrated in Fig. 3, in which are utilized three-electrode thermionic devices of the usual and well-known type while perfectly oper- 1 able to produce the desired result, has the slight disadvantage of necessitating two separate sources of filament power, one for the receiving apparatus proper and another for the control tube. In order to overcome this diiliculty, I may also use thermionic devices of the equipotential cathode type, such, for example, as those shown in the patent to Nicolson, 1,459,412. In this modification of my invention (not shown) the heater elements of all of the tubeslmay be fed from a single secondary winding, inasmuch as the tubes may be so manufactured that the said elements are thoroughly insulated from the cathodes. The cathodes of all of the tubes may be connected to the same points in the circuit network that the filaments are shown as connected to in Fig. 3, no other alterations being necessary to change the system from direct to alternating current operation.

If it is desired to substitute manual for automatic volume control, the switch device I33 is closed and the resistor I2! is thereby connected in shunt to that portion oi the voltage source I I I included between conductors I28 and I29. By manually adjusting the movable element IIB along the resistor, the grids of the radio fre quency amplifier tubes and the grids of the intermediate frequency amplifier tubes may be given a potential varying from zero to the potential of the point on the voltage source to which the conductor I29 is connected, which point has a potential of minus 20 volts.

In the event that economy of construction is a major consideration, rather than maximum efliciency, the control-tube I22 and its associated circuits may be omitted, and the control-voltage derived directly from aresistor positioned in series in the plate circuit of the second detector tube. Inasmuch as the necessity for maintaining the cathode of the control tube at a high negative potential with respect to the cathodes of the tubes controlled thereby has been shown, if the control-tube is replaced by the second detector-tube, such tube must either be supplied from a separate A-battery, or must be of the equipotential, or heater-cathode, type. If of the latter type, the heater-winding, 'or filament, may be supplied with energy from the same A-ba'ttery that is utilized for the balance of the tubes, since the cathode is thoroughly insulated therefrom.

In order that the aforementioned modification of my invention may be made clear, reference should now be made to Fig. 4, which is a diagram-- matic view of a portion of a superheterodyr'ie re-- plies cathode energy for both the second detector tube, and the remaining amplifier tubes of the .system, including the tube I55.

A single source ISI of plate and grid-biasing potential is provided, to an intermediate point I62 on which is connected the filaments of the amthe plate potential source is assumed to be at zero potential, and is the reference point from which he various potentials applied to the amplifier t has are measured.

The cathode I52 of the detector-tube I50 is connected by a conductor I65 to a-point I66 on the source I6I which has a negative potential of approximately 220 volts, and the grid I53 of the same tube is connected to a point I68 by a conductor IGI, having a negative potential of approximately 250 volts, thus placing a negative bias of approximately 30 volts on the grid. The plate I54 is connected to a point I69 having a negative potential of about 9 volts, thus impressing a positive potential of about 211 volts thereon with respect to the cathode I52.

A resistor III! is interposed in the plate circuit of the second detector-tube, and the grid' I51 of the amplifier tube is connected to an appropriate point Ill thereon by a conductor I12. Inasmuch as the resistor I10 connects to a point on the source IBI which has a negative potential 9 volts with respect to the cathode of the amplifier tube, the grid of the latter tube is normally negatively biased to that extent. In some instances it may be desirable to positively bias the grid of the amplifier tube with respect to its cathode, and in such event the resistor is connected to a point on the source IGI which is positive with the respect to the point I62. All of the voltages specified are approximate, and they will, of course, vary accord ng to the type of tubes utilized.

The operation of the modification of my invention just described is substantially the same as the operation of the modificationillustrated in Fig. 3. Variations in the plate current of the detector-tube give rise to yariations in the resistance voltage drop in the resistor I10 and cause the potential of the amplifier tube grid to be so changed with respect to the filament thereof that the effective amplification in said tube is altered. For example, if an incoming signal, by reason of fading-in, or from any other cause, gradually increases in strength, the output of the detector tube increases proportionately, and the point on the resistor III to which the grid of the amplifier 1 tube is connected assumes a more negative potential, and the effective amplification therein is reduced. If the incoming signal fades-cu or diminishes in strength, the plate current of the detector tube is decreased, and the grid of the amplifier tubebecomes more positive, thus increasing the effective amplification therein.-

The principal advantage of my invention lies in the fact that it permits a multi-tube amplifier to operate with a substantially even sound-output, irrespective of the fading or changing in average amplitude of an incoming signal. This condition is very desirable in connection with receiving apparatus utilizing power amplifier tubes, inasmuch as the effect of varying signal amplitude is greatly accentuated thereby;

My invention has a further advantage in that it prevents the power amplifier tubes from being overloaded and consequently, injuring the sound reproducer.

Although I have illustrated and described only a few specific embodiments of my invention, numerous modifications thereof will be apparent to those skilled in the art. My invention, therefore, is not to be limited except insofar as is necessitated by the prior art or by the spirit of the appended claims.

I claim as my invention:

1. In combination, an amplifier comprising an electron tube device, a second tube device having a resistor in the output circuit thereof, means for supplying anode potential to said second device, continuously variable means for negatively biasing the grid 01' said second device to fix the output level of the amplifier, means for impressing signals on both of said electron tube devices, and means connecting said resistor with the grid of said first-mentioned triode whereby a tendency toward an increase .in average signal amplitude results in a tendency for said grid to assume a more negative potential than it possessed previous to said amplitude-increase tendency.

2. In an amplifier, manually operable means for controlling the volume of the output therefrom, automatic means for controlling the volume irrespective of the amplitude of incoming signals, both of said means having elements in common, and manually operable means for selecting the type of control desirable.

3; In combination, an amplifier comprising 11. thermionic device arranged to amplify high frequency alternating currents, an electric discharge device connected with said thermionic device to control the effective amplification therein, means for impressing the high frequency alternating potential on said discharge device, and continuously variable means for rendering said discharge device responsive topotentials of any predetermined amplitude, whereby electric discharge devices of differing characteristics may be utilized and the output level of the amplifier may be preset at any desired value.

4. In combination, an amplifier comprising a thermionic device arranged to amplify high frequency alternating currents, a second thermionicdevice having a grid, means connected with said grid to impress said high frequency alternating currents thereon, said second thermionic device being connected with said first mentioned device to control the eifective amplification therein, and continuously variable means whereby the potential of the grid of the second thermionic device may be adjusted relative to the filament thereof.

5. An amplifier having grid and anode electrodes, means for impressing signals on said amplifier, means responsive to variations in the average amplitude of said signals for controlling the amplification of said amplifier, said last means, comprising an electric discharge device having grid and anode electrodes, and common potential supply means electrically connected to said anodes and at least one of said grid electrodes for supplying said amplifier and control means with anode and bias potentials.

6. An amplifier comprising a plurality of thermionic devices having grid and anode-electrodes, a thermionic device of the equipotential type for controlling the amplification of said amplifier, said thermionic device having grid and anode electrodes, and potential supply means common to said anodes and at least one or said grids for supplying anode and grid potentials to said amplifier and control device.

7. An amplifier comprising a plurality of thermionic devices, means for impressing signals successively on said devices, signal-responsive means comprising an electron discharge device for controlling the effective amplification in certain of said devices, and potential supply means common to said amplifier and said control means for supplying the thermionic amplifying devices and the signal responsive means with plate potential and grid-biasing potential.

8. In combination, a plurality of signal amplifying thermionic devices connected in cascade, a-

signal, responsive thermionic device for controlling the effective amplification of said amplifying devices, a common source of grid biasing and plate potentials for said devices, and connections whereby the cathode of said signal-responsive device is maintained at a high negative potential relative to the cathodes of said amplifying devices.

9. In combination, a plurality of signal-amplifying thermionic devices connected in cascade, a signal-responsive thermionic device for controlling the efiective amplification of said amplifying devices, a common source of grid biasing and producing means, control means actuated by an incoming signal derived from one of said amplifying stages, means connecting said control-means to one of said amplifying stages subsequent to the amplifying stage from which said signal was derived, and means providing a common source of anode current and biasing potential for said amplifying stages and said controlling means.

11. In an amplifier comprising a plurality of thermionic devices connected in cascade, said devices having input and output circuits, means for deriving a biasing potential from signal currents, said means also having input and output circuits, means for selectively coupling the input circuit of said bias potential deriving means across the input circuits of said thermionic devices, and means for selectively coupling the output circuit of said bias potential deriving means to said thermionic devices whereby said potentials may be applied to any one of said thermionic devices to control the degree of amplification therein.

12. In a radio receiving system, means for providing automatic volume-control and means, in cluding a portion of said first-named'means for simultaneously removing said automatic volume 14. In combination, an amplifier comprising a plurality of electron discharge devices one of which is arranged and connected to function as a detector, a second detector device having an output circuit, means for impressing signal voltages simultaneously on the input circuits of both of said detector devices, said output circuit being coupled to input circuits of certain of said electron discharge devices preceding and following said first mentioned detector whereby changes in the current in said output circuit are efiective to cause alterations in the grid potentials of said devices in said amplifier.

15. In a carrier current signalling system, an amplifier for said carrier current, means for automatically controlling the amplification of said amplifier in response to the variations in level of the received carrier, and means other than said automatic amplification controlling means for manually controlling the normal sensitivity of said .amplifier independently of said automatic volume control means, said automatic volume control means and said manually adjustable sensitivity control means being connected in circuit between the same electrodes of an electron discharge device in said signalling system.

16. In a carrier current signalling system, an

amplifier for said carrier current, means for automatically controlling the amplification of said amplifier in response to the level of the received carrier, and means other than said automatic amplification controlling means for manually controlling the normal sensitivity of said amplifier independently of said automatic volume control means, said automatic volume control means and said manually adjustable sensitivity control means being connected in series.

1'7.'In a radio receiving system comprising an amplifier having a plurality of elements which contribute to the selectivity of said amplifier, means connected to an element of said amplifier for deriving a modulation frequency current from a received signal, automatic volume control means including a rectifier device for controlling the sensitivity'of said amplifier, and means providing a lesser degree of selectivity from the input of said amplifier to said automatic volume control rectifier device than to said modulation current deriving means.

18'. In a modulated carrier wave receiving system, aplurality of carrier current selector circuits connected in cascade, means responsive to variations in carrier current amplitude for automatically varying. the carriercurrent transmission through the circuits, said last named means including a "carrier current rectifier, a demodulator coupled to the lastof said circuits, and means for increasing the ratio of the carrier wave potential applied to said carrier current rectifier to that applied to said demodulator, as the receiving system is tuned off-resonance from a desired carrier wave.

19. In a radio receiving system, a plurality of amplifying stages in cascade, means for deriving a modulation frequency current therefrom, control means actuated by an incoming signal derived from one of said amplifying stages to pro vide a gain-control potential, and means connecting said control means to apply said gaincontrol potential to at least one of said amplifying stages subsequent to,. and preceding, the amplifying stage from which said signal for said control means was derived.

20. In a carrier current signalling system, an amplifier for said carrier current, means for automatically controlling the amplification of said amplifier in response to the level of the received carrier, and means other than said automatic amplification controlling means for manually controlling the normal sensitivity of said amplifier independently of said automatic volume control means.

21. In a radio receiving system, an amplifier circuit followed bya detector circuit, said receiving system being less selective at said amplifier stage than at said detector, and an autocontrolling the normal sensitivity of saidamplifier circuit to derive control potentials therefrom, the coupling for said automatic volume control circuit with said amplifier circuit providing a lesser degree of selectivity than the selectivity of said receiving system at the detector circuit.

22. In a radio receiving system, a detector circuit, an amplifier circuit preceding said detector circuit, an automatic volume control circuit and means for coupling said automatic volume control circuit to said amplifier to derive control potentials therefrom and providing less selectivity than said detector circuit, said lastnamed means providing a lesser degree of selectivity in said' 23. In a carrier current signalling system, an amplifier for said carrier current, means for automatically controlling the amplification of said amplifier in response to the level of the received carrier, means other than said automatic amplification controlling means for manually controlling the normal sensitivity of said amplifier independently of said automatic volume control means, said automatic controlling means including a source of variable biasing potential and said manual controlling means including a fixed source of biasing potential and manually operable means for deriving a variable biasing potential from said source, and means for selectively causing the fixed or the variable source of biasing potential to become effective to provide the main controlling biasing potential for said amplifier.

GEORGE L. BEERS. 

